1. Field of the Invention
This invention relates to an apparatus for detecting and selecting between progressively scanned signals and interlaced signals.
2. Description of the Related Art
Historically, television broadcast signals are transmitted and received in an interlaced form such that, in a first field, odd numbered lines of a picture are firstly transmitted followed by even numbered lines so that two transmitted fields are subsequently interlaced to make up a frame of a television picture. In such a system, an interlaced display at a receiver is normally a cathode ray tube. However, with the use of personal computer (PC) systems, pixels on a display are scanned and energised progressively in, for example, a line-by-line fashion using a PC graphics card and, for example, a plasma display. There is an existing trend away from interlaced displays towards progressive displays for consumer viewing where consumers are not only viewing films and matter downloaded or uploaded into the PC, but also watching normally broadcast television on a PC monitor display.
Such progressive displays associated with PC systems may be, for example, liquid crystal displays (LCD) or plasma displays. However, because of differences between an interlaced display and a progressive display, so, in practice, progressive displays are not able to display interlaced material without aberrations. Such aberrations are called “interlace artefacts” which appear wherever there is motion and, in particular, fast motion. These artefacts take the form of zig-zag jagged edges of moving objects. Such artefacts are annoying to a viewer and, in systems which are set up to purely display interlaced signals in a progressive manner on a progressive display, so a process known as de-interlacing is used and such a process is described in EP-A-0160063.
In present day broadcasts it is often required to insert progressive, film-type sequences into an interlaced signal, e.g. where an advertisement is inserted into a conventionally scanned interlaced broadcast programme.
It has long been known in the tele-cine art that it is possible to convert 24 frames per second video into 30 frames per second video signals in a process called 3:2 pull down. This process repeats every fourth field to thereby convert film into a picture quasi-interlaced format. Since the repeated fields are readily detectable, so transmitted broadcast signals are easily reversed back to their original format at a receiver. Thus, most encoders reverse the 3:2 pull down structure back to the original 24 progressive frames per second. In order to convert 24 frames per second into 25 frames per second video, the 24 frame per second film is played out 4.17% faster so as to provide 25 frames per second. By such an expedient, each film frame is converted into two consecutive video fields, thereby providing a one-to-one match between film frames and video frames. Computer graphics, cartoon videos and many other sources are typically generated using progressive frames with 25 or less motion cycles per second. Difficulty arises where such progressively derived material is intermingled with interlaced (TV camera) video signals with 50 motion cycles per second.
It is known from Signal Processing: Image Communication, an article by A. Bock “Motion Adaptive Standards Conversion Between Formats Of Similar Field Rates”, 10th Dec., 1993, publisher Elsevier, to pass a progressive by scanned signal through a de-interlacer. Such a motion-adaptive de-interlacer is shown in block schematic form in FIG. 1.
Referring to FIG. 1, an interlaced video signal on an input 1 is applied to a motion detector 2, a vertical filter 3 and a temporal filter 4. The motion detector provides a motion signal output 5 which controls a switch 6 that selects between output 7 of the vertical filter and output 8 of the temporal filter provides a progressive video signal output 9. Thus, the interlaced video signal input undergoes two types of filtering process. The vertical filter 3 samples the current field only so as to provide intra-field filtering thereby interpolating video samples in areas of fast motion. The temporal filter 4 is used to provide inter-frame filtering so as to interpolate video signals in area of little or no motion. The motion detector 2 selects the appropriate filtered signal on a pixel-by-pixel basis. Because the motion detector detects frame differences between video frames, the vertically filtered signal is used in moving picture areas even if the applied signal is changed to the progressive format. Such filtering of a progressively scanned signal produces a lower vertical resolution at the output 9 than the applied signal at input 1.
Thus, applying a progressively scanned signal to a de-interlacer produces a lower vertical resolution at output 9 than the applied signal at input 1 when viewed on a progressive display. As described above, applying an interlaced signal to a progressive display without de-interlacing produces annoying artefacts. Applying an interlaced signal through a de-interlacer produces a higher vertical resolution than the applied signal when viewed on a progressive display.
If compression is used, de-interlacing the video signal before encoding has the added advantage that it saves bit rates, since most compression algorithms such as MPEG-2 and H.264 are more efficient on progressive than on interlaced video signals.
It is to be understood that the interlacing may be performed either at an encoder or a decoder.
De-interlacing usually uses some form of vertical-temporal filtering which has the disadvantage of not only modifying interlaced frames, but also modifies progressive frames. The net result tends to be a reduction in vertical resolution of progressive frames where there is motion.